This invention relates to a cleaning appliance and, more particularly, to a dishwasher including a sensing mechanism for determining the turbidity of fresh fluid added to the appliance and of the fluid at the conclusion of various operating cycles of the appliance.
There is significant need to reduce the energy consumed by appliances such as domestic dishwashers. More specifically, water used in such dishwashers is heated prior to its introduction into the machine and many such appliances include auxiliary heaters that further heat the fluid in the machine during a washing sequence. Thus it is desirable to minimize the number of separate operating cycles performed in a complete washing sequence. In the past such dishwashers have provided the user with the ability to choose between sequences including different predetermined numbers of operating cycles, often based upon whether the machine was to wash a load of dishes and tableware, a load of cooking utensils or a mixed load of items. Also many past machines permitted the user to select between different washing sequences based upon the user's estimate of how soiled the items were. If the user guessed incorrectly either the items were not sufficiently cleaned or the machine performed too many operating cycles, with an attendant waste of water and heat energy. Typically, users choose an operation sequence which would assure that the items were cleaned, which meant that many sequences included too many cycles and wasted water and energy.
More recently dishwashers have been designed which included devices for measuring the turbidity of the fluid in the dishwasher and controlled the number and length of the operating cycles based upon the condition of the fluid. One such system is shown and described in co-pending application Ser. No. 08/370,752 filed Jan. 10, 1995, herein incorporated by reference. The turbidity sensing mechanisms incorporated in many such machine designs attempt to measure the turbidity of the fluid when it is in a dynamic state. Such measurements are difficult to obtain and tend to be unreliable for a number of reasons. For example, the fluid in a dynamic state will tend to have bubbles entrained, which distorts the turbidity measurements. Furthermore turbidity sensing mechanisms are subject to measurement error due to many factors such as light source dimming or component performance degradation with age.
It is desirable to provide a dishwasher with a turbidity sensing system and mechanism that senses the turbidity of the fluid when the fluid is quiescent and provides compensation for factors that cause measurement errors with a sensing system. It also is desirable to provide a dishwasher with a sensing mechanism that is simple and economical to produce.